A key feature of the Senate bill is that it would make illegals pay back taxes before applying for citizenship, a requirement that supporters say will raise billions of dollars in the next decade.

There’s just one problem: The U.S. Constitution specifically prohibits revenue-raising legislation from originating in the Senate.

“All bills for raising revenue shall originate in the House of Representatives,” according to the “origination clause” in Article I, Section 7.

Republicans — including the bill’s supporters — say this will kill the bill, and Senate Majority Leader Bill Frist says he’s offered a simple solution. He wants to attach the immigration bill to a tax bill that has already passed the House. It would then proceed as planned to a “conference committee,” where negotiators from the House and Senate hammer out differences between the two chambers’ immigration bills.

“This is a procedural issue that we could overcome,” said Carolyn Weyforth, spokeswoman for Mr. Frist.

But Minority Leader Harry Reid won’t go along with that fix. His office said yesterday that the concerns raised by Mr. Frist and House Republicans are “technical in nature” and can be ignored.

“If Republicans are serious about enacting comprehensive immigration reform, I’ve got a deal for them,” spokesman Jim Manley said. “All they have to do is nothing. Just let the House and Senate bills go to conference and let the conferees work their will.”

The bill as written, however, will never make it to conference, Republicans say. Under House rules, any member can introduce a “blue-slip resolution” to return the legislation to the Senate. And although there are plenty of House conservatives eager to kill the Senate bill any way they can, Hill staffers say it would likely be done based on “policy-blind constitutional issues.”

“If there is a blue-slip issue, it is not about policy,” said one House aide familiar with the matter. “It’s about procedure and the House’s prerogative to uphold the United States Constitution.”

There is some debate about whether the constitutional ban on Senate revenue bills pertains only to bills whose primary purpose is to raise money or whether it also applies to bills that do so incidentally.

Republicans say that because the concerns already have been raised, they expect the bill to be returned to the Senate.

They point to a 2002 report written by the nonpartisan Congressional Research Service that reveals how broadly the House interprets the origination clause.

“In 1992, the House returned to the Senate a bill (S.884, 102nd Congress) to require the President to impose economic sanctions, including a ban on certain imports, against countries which fail to eliminate large-scale driftnet fishing,” according to the report. “In 1999, the House returned to the Senate a bill (S.254, 105th Congress) effectively banning the import of certain assault weapon attachments.”

The policy of the House, according to the report, has been to interpret “any meaningful revenue proposal” as subject to the origination clause.